1. Field of the Invention
The present invention relates to wireless process control systems and methods, and more particularly to such systems and methods that include hierarchical adaptability to operate a wireless process control and/or automation network while utilizing minimum system resources.
2. Description of Related Art
The International Society of Automation (ISA) has established a Wireless Systems for Automation Standards Committee (ISA-SP100) tasked with defining wireless connectivity standards. The SP100 wireless standard for process automation systems is applicable to industries such as oil and gas, petrochemical, water/wastewater treatment and manufacturing. The SP100 standard is intended for use in the 2.4 GHz band, with data transfer at speeds up to 250 kilobytes per second within a 300 meter range. SP100 devices have relatively lower data rates and energy requirements than comparable wireless Local Area Networks (LAN), as they are intended to be low cost devices.
The SP100 protocol specifies different types of communications, categorized as “usage classes,” and increasing in criticality based upon decreasing numerical designation. “Class 0” communications include those categorized as critical for safety applications such as emergency shut-down systems, and are deemed always critical; “Class 1” is for closed-loop regulatory control, often deemed critical; “Class 2” is for closed-loop supervisory control, usually non-critical; “Class 3” is for open-loop control; “Class 4” is for alerting or annunciation; and “Class 5” is for data logging. Certain events, such as alarms, can have different classifications of service depending on the message type.
In general, devices in an SP100 system can be divided into three categories, commonly referred to as “tiers.” Tier 1 includes end devices, such as meters, remote terminal units, valves, sensors, tank level measuring devices, and the like, each of which is connected to a wireless end device. Wireless end devices (WEDs) can transmit to and receive from all other devices, but cannot route to other devices. Tier 2 includes wireless intermediate devices (WIDs), which transmit to and receive from all other devices, and route to other devices. Tier 3 includes wireless gateway devices (WGDs), which transmit to, receive from, and route between other devices, and also conduct high level applications including protocol translation and assignment of paths for source-destination pairs. As used herein, the components WEDs, WIDs and WGDs are also referred to as “nodes.”
FIG. 1A is a schematic diagram of a known exemplary architecture for an SP100 Wireless Process Control System of the prior art. Connectivity between WEDs L17 and L13 and WGDs L35 and L31, respectively, are illustrated, although as will be understood by one of ordinary skill in the art, connectivity is typically provided between all WEDs and a WGD at the Central Control Room (CCR). For example, L17-L293-L292-L36-L35 is a path for the source-destination pair L17-L35, and L292-L35 is one of the links within this path.
Devices in an SP100 wireless system are generally connected in the form of a mesh or star-mesh network. Connection between the various devices is performed through radio communications, for instance as specified by a Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) protocol or the like, and connections are established at a network layer and a Medium Access Control (MAC) layer.
In existing wireless process control and/or automation systems, every frame transmitted from WED to the CCR is treated the same, regardless of its usage class or criticality. The constraints are that the transmitted frames reach the CCR within specified maximum allowable end-to-end time delay and a specified frame error rate (FER). Commonly, all WIDs and WGDs route incoming traffic irrespective of the usage class, and without regard to a frame's status as an original transmission or a retransmission. Multiple paths between WEDs and the CCR are typically specified in a routing table for increased reliability of data frame transmission and receipt. Retransmission of frames occurs and is requested if the received frame is judged to be erroneous or no acknowledgment is received (i.e., timeout occurs).
While a large number of paths provide a certain degree of reliability, this topology increases the bandwidth requirements for the wireless spectrum, battery energy usage, and quantity and/or sophistication level of requisite hardware. In addition, channel contention often occurs due to high channel utilization, increased latency between the WEDs and CCR, and frame blocking. Therefore, diminishing returns result, such that an increase in the number of paths beyond a certain level will not significantly increase the reliability, thereby inefficiently using bandwidth, hardware and battery usage energy requirements.
Another commonly employed wireless process control and/or automation network has been recently developed as a derivative of the Highway Addressable Remote Transmitter (HART) Communication Foundation protocols, referred to generally as the HART® protocol. However, the wireless implementation of the HART® protocol has suffered some of the same drawbacks as the SP100 protocol, namely, battery usage and channel contention.
Therefore, a need exists for reliable and adaptable methods and systems to operate a wireless process control and/or automation network while utilizing minimum system resources.